1. Field of the Invention
This invention relates in general to a fluorescent lamp, and more particularly, to a planar fluorescent lamp that can be used as the backlight for a large area liquid crystal display (LCD).
2. Description of the Related Art
Having the advantages of high image quality, small volume, low driving voltage, low power consumption and a wide range of application, the liquid crystal display has been broadly applied to consuming products to replace the conventional cathode ray tube (CRT). The application of the liquid crystal display includes medium-and small-size portable television, cellular phone, camcorder, notebook computer, desktop computer, projection-type television and other computer products. However, different from the self-illuminating type display such as the plasma display panel (PDP), electro-luminescent apparatus, and light emitting diode, the liquid crystal display is a light accepting apparatus that requires an external light source to achieve the display effect. That is, most of the liquid crystal displays requires a backlight behind the display panel.
The typical backlight of the liquid crystal display includes a fluorescent lamp. Currently, the caliber of the fluorescent lamp is between 1.8 mm to 2.6 mm. The structure of the fluorescent lamp includes electrodes at two sides of a glass tube, while the interior wall of the glass tube is coated with phosphor. The glass tube is filled with mercury vapor and inert gas. By applying a voltage to the electrodes, electrons are generated to bombard the mercury vapor and inert gas, which are then agitated to an excited state. When the mercury vapor and the inert gas returns to the ground state, an ultra-violet light is emitted to excite the phosphor to generate a visible light.
As the display area of the liquid crystal display gradually increases, a planar illumination source able to emit a white light with a uniform brightness is required. However, the white fluorescent lamp is a non-planar line light source. The most direct way is to install several fluorescent lamp tubes behind the display panel. Referring to FIG. 1, a cross sectional view of an array-type back light is shown. The fluorescent lamp 100 is installed at a rear surface of the liquid crystal display panel 102 in parallel. A reflector 104 is located behind the fluorescent lamp 100. A diffuser 106 is located between the fluorescent lamp 100 and the display panel 102 to obtain the effect of a light source.
In another approach to transfer a linear light source into a planar light source, a fluorescent lamp is installed at a terminating surface of a louver to obtain the effect of a light source by edge light. Referring to FIG. 2, a cross sectional view of an edge-light-type backlight is shown. The fluorescent lamp 200 is installed on a terminating surface 202a of a light-guide board 202. A reflector 204 directs the light emitted from the fluorescent lamp 200 to the light-guide board 202 made of acrylic. A front surface of the light-guide board 202 has a diffuser 206, and a rear surface and other terminating surfaces are covered with the reflector 204, such that the light directed into the light-guide board 202 is restricted. The light entering the light-guide board 202 is reflected several times until the light-guide board 202 emits as a planar light source. The diffuser 206 is used to uniform the light emitted from the light-guide board 202.
However, the array-type back-light requires a diffuser to uniform the overall brightness thereof. When the fluorescent lamp is too close to the display panel, the profile thereof is displayed on the liquid crystal display panel to affect the display quality. Adjusting the distance between the fluorescent lamp and the display panel increases the thickness of the backlight. The liquid crystal display cannot be thinned as required. Generally speaking, the edge-light back light has a brightness uniformity superior to that of the array-type back light. However, the brightness of the edge-light type is smaller due to a poorer application efficiency of light. To solve the problem, the planar fluorescent light is used as the light source for a liquid crystal display. The current planar fluorescent lamp as shown in FIG. 3 includes two parallel glass panels 300, 302 with a glass rim 304 in between. A venting orifice 305 is located at one side of the glass rim 304 for vacuum and gas introduction. Electrodes 306 are formed in a recess 308 of the glass rim 304. The electrode leads 310 are solderly joined with the electrodes 306 to connect an external operating circuit. As the electrodes 306 are parallel to each other, the solder joint between the electrode leads 310 and the electrodes 306 has to be twisted with an angle approximate to a right angle. Thus, the electrode leads 310 occupy a significant area to reduce the illuminating area of the fluorescent lamp.
The metal for forming the electrodes in the planar fluorescent has a thermal expansion coefficient far different from that of glass. To obtain a hermetic planar fluorescent, a metal with an expansion coefficient close to that of glass is required for forming the electrode lead.
In the fabricating process of the fluorescent lamp, the yield is frequently reduced due to the difference in thermal expansion coefficient between the electrodes and the electrode leads. The fabrication cost is thus increased.
The electrodes assembly of the planar fluorescent is directional, and is inconvenient for automatic production. The fabrication of electrode is complex. Being restricted with the planar electrodes, the thinning process of the planar fluorescent lamp is affected.
In addition, a critical point of the electrode surface is caused by edge comer of the planar electrodes. While assembling the planar electrodes, fluorescent layer on the panel is easily scratched by the protruding edge.